1. Field of the Invention
The invention concerns neuroprotein changes associated with neurological damage and particularly to assays and diagnosis relating to traumatic brain injury.
2. Description of Background Art
The incidence of traumatic brain injury (BI) in the United States is conservatively estimated to be more than 2 million persons annually with approximately 500,000 hospitalizations. Of these, about 70,000 to 90,000 head injury survivors are permanently disabled. The annual economic cost to society for care of head-injured patients is estimated at $25 billion. Assessment of pathology and neurological impairment immediately after TBI is crucial for determination of appropriate clinical management and for predicting long-term outcome.
The outcome measures most often used in head injuries are the Glasgow Coma Scale (GCS), the Glasgow Outcome Scale (GOS), computed tomography and magnetic resonance imaging (MRI) to detect intracranial pathology. However, despite dramatically improved emergency triage systems based on these outcome measures, most TBI patients suffer long term impairment and a large number of TBI survivors are severely affected despite predictions of “good recovery” on the GOS. In addition, CT and MRI are expensive and cannot be rapidly employed in an emergency room environment. Moreover, in austere medical environments associated with combat, accurate diagnosis of TBI would be an essential prerequisite for appropriate triage of casualties.
The neural pathways of a mammal are particularly at risk if neurons are subjected to mechanical or chemical trauma or to neuropathic degeneration sufficient to put the neurons that define the pathway at risk of dying.
TBI represents a major central nervous system (CNS) disorder without any clinically proven therapy. Evidence of axonal damage following TBI is recognized and prolonged traumatic axonal injury (TAI) is a universal and critical event following TBI, as well as a key predictor of clinical outcome. Integrity of myelin sheaths, which surround axons, is not well studied, but has been reported to increase after TBI in humans.
Collapsin response mediator protein-2 (CRMP-2), also known as CRMP62, TOAD-64 (turned on after division 64 kDa), Ulip-2 (Unc-33-like phosphoprotein) and DRP2 (dihydropyrimidinase-related phosphoprotein), is one of at least five members (CRMP-1-5) of the CRMP family. It was first identified as an intracellular component of the extracellular semaphoring 3A (Sema 3A) signal transduction pathway in chick dorsal root ganglia (DRG), which was known as an inhibitor protein for axonal guidance. CRMP-2 is a developmentally regulated protein that is exclusively expressed in the nervous system. It is concentrated in growing axons, dendrites, and the cytoplasm of differentiating neurons.
A lesser amount of CRMP2 has been detected in select adult neurons, such as the pyramidal cells of the hippocampus, Purkinje cells of the cerebellum and sensory neurons of the DRG. CRMP-2 appears to have an important role in the determination of axon and dendrite integrity. Inagaki, et al. (2001) initially found enrichment of CRMP2 in the distal parts of growing hippocampal axons but later discovered that over-expression of full-length CRMP2 induced formation of multiple axons and elongation of the primary axon, while the dominant-negative form of CRMP2 inhibited axon formation in hippocampal cell culture. The presence of CRMP2 fosters conversion of immature neurites and preexisting dendrites into axons.
Non-phosphorylated CRMP2 enriches in axonal growth cones, promotes axon outgrowth, and induces formation of multiple axon-like neurites. GSK-3-phosphorylated CRMP2 at Thr-514 inactivates CRMP2 and thereby inhibits neuronal polarization. Neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) inhibits GSK-3b via the phosphatitylinositol-3-kinase (PI3-kinase)/Akt (also known as PKB) pathway and thereby reduces phosphorylation levels of CRMP2 at Thr-514, leading to axon elongation and branching.
A high degree of phosphorylation is associated with neurofibrillary tangles in Alzheimer's diseased brains, suggesting that CRMP2 may play a role in neurodegeneration. A growing body of evidence suggests that CRMP2 may also participate in the pathophysiology of other neurological disorders. Decreased expression of CRMP2 has been reported in fetal brains with Down's syndrome, patients with mesial temporal lobe epilepsy, focal ischemic rat brain and in the frontal cortex of patients who suffer from psychiatric disorders such as schizophrenia, bipolar, or major depression disorders. In contrast, an increase in CRMP2 is observed after chronic anti-depressant treatment in rat hippocampus. CRMP2 has also been reported to mediate axonal damage and neuronal death via a semaphorine-CRMP pathway.
The role, if any, of synaptic dysfunction in relation to neural injury or brain trauma is not well understood. Synaptotagmins are important calcium sensor proteins that allow the docking of synaptic vesicle onto the presynaptic terminal, thus initiating the neurotransmitter release process. Yet, the role and fate of synaptotagmins following TBI is unknown. In contrast, proteolysis of axonal proteins such as neurofilament proteins, amyloid precursor protein (APP) and αII-spectrin following TBI has been documented.